The following description provides a summary of information relevant to the present invention. It is not an admission that any of the information provided herein is prior art to the presently claimed invention, nor that any of the publications specifically or implicitly referenced are prior art to that invention.
Definitions
The following descriptions of the inventions contained herein use numerous technical terms specific to the field of the invention. Generally, the meaning of these terms are known to those having skill in the art and are further described as follows:
As used herein, “sample” refers to a substance that is being assayed for the presence of one or more nucleic acids of interest. The nucleic acid or nucleic acids of interest may be present in a mixture of other nucleic acids. A sample, containing the nucleic acids of interest, may be obtained in numerous ways. It is envisioned that the following could represent samples: cell lysates, purified genomic DNA, body fluids such as from a human or animal, clinical samples, food samples, etc.
As used herein, the phrases “target nucleic acid” and “target sequence” are used interchangeably. Both phrases refer to a nucleic acid sequence, the presence or absence of which is desired to be detected. Target nucleic acid can be single-stranded or double-stranded and, if it is double-stranded, it may be denatured to single-stranded form prior to its detection using methods, as described herein, or other well known methods. Additionally, the target nucleic acid may be nucleic acid in any form most notably DNA or RNA.
As used herein, “amplification” refers to the increase in the number of copies of a particular nucleic acid target of interest wherein said copies are also called “amplicons” or “amplification products”.
As used herein, “amplification components” refers to the reaction materials such as enzymes, buffers, and nucleic acids necessary to perform an amplification reaction to form amplicons or amplification products of a target nucleic acid of interest.
As used herein, the phrase “multiplex amplification” refers to the amplification of more than one nucleic acid of interest. For example, it can refer to the amplification of multiple sequences from the same sample or the amplification of one of several sequences in a sample, as described in U.S. Pat. Nos. 5,422,252 and 5,470,723 which are incorporated herein by reference. The phrase also refers to the amplification of one or more sequences present in multiple samples either simultaneously or in step-wise fashion.
As used herein, “oligonucleotide” refers to a molecule comprising two or more deoxyribonucleotides or ribonucleotides, preferably more than three. The length of an oligonucleotide will depend on how it is to be used. The oligonucleotide may be derived synthetically or by cloning. Oligonucleotides may also comprise protein nucleic acids (PNAs).
As used herein, “probe” refers to a known sequence of a nucleic acid that is capable of selectively binding to a target nucleic acid. More specifically, “probe” refers to an oligonucleotide designed to be sufficiently complementary to a sequence of one strand of a nucleic acid that is to be probed such that the probe and nucleic acid strand will hybridize under selected stringency conditions. Specific types of oligonucleotide probes are used in various embodiments of the invention. For example, a “ligation probe” describes one type of probe designed to bind to both a target nucleic acid of interest and to an amplification probe. Additionally, a “ligated probe” or a “ligated probe template” refers to the end product of a ligation reaction between a pair of ligation probes.
As used herein, the terms “primer molecule” and “primer” are used interchangeably. A primer is a nucleic acid molecule with a 3′ terminus that is either “blocked” and cannot be covalently linked to additional nucleic acids or that is not blocked and possesses a chemical group at the 3′ terminus that will allow extension of the nucleic acid chain such as catalyzed by a DNA polymerase or reverse transcriptase.
As used herein, the phrase “amplification primer” refers to an oligonucleotide primer used for amplification of a target nucleic acid sequence.
The phrase “primer extension,” as used herein refers to the DNA polymerase induced extension of a nucleic acid chain from a free three-prime (3′) hydroxy group thereby creating a strand of nucleic acid complementary to an opposing strand.
As used herein, the term “amplicon” refers to the product of an amplification reaction. An amplicon may contain amplified nucleic acids if both primers utilized hybridize to a target sequence. An amplicon may not contain amplified nucleic acids if one of the primers used does not hybridize to a target sequence. Thus, this term is used generically herein and does not imply the presence of amplified nucleic acids.
As used herein, “electronically addressable” refers to a capacity of a microchip to direct materials such as nucleic acids and enzymes and other amplification components from one position to another on the microchip by electronic biasing of the capture sites of the chip. “Electronic biasing” is intended to mean that the electronic charge at a capture site or another position on the microchip may be manipulated between a net positive and a net minus charge so that charged molecules in solution and in contact with the microchip may be directed toward or away from one position on the microchip or from one position to another position.
As used herein, the phrase “capture site” refers to a specific position on an electronically addressable microchip wherein electronic biasing is initiated and where molecules such as nucleic acid probes and target molecules are attracted or addressed by such biasing.
As used herein, the term “anchored” refers to the immobilization by binding of a molecule to a specified location on a microchip, such as a primer nucleic acid used in an SDA reaction, or a nucleic acid probe used to capture a target nucleic acid.
As used herein, the term “branched primer pair” refers to a pair of oligonucleotides that may be used as primers in an amplification reaction and which are connected together through a chemical moiety such that the oligonucleotides are susceptible to hybridization and use as amplification primers.
As used herein, the term “primer capture probes” refers to oligonucleotides that are used to hybridize to selected target nucleic acids and provide anchoring support for such nucleic acids to a capture site. Moreover, such oligonucleotides may function as amplification primers for amplifying said target nucleic acids.
As used herein, “hybridization” and “binding” are used interchangeably and refer to the non-covalent binding or “base pairing” of complementary nucleic acid sequences to one another. Whether or not a particular probe remains base paired with a polynucleotide sequence depends on the degree of complementarity, the length of the probe, and the stringency of the binding conditions. The higher the stringency, the higher must be the degree of complementarity, and/or the longer the probe for binding or base pairing to remain stable.
As used herein, “stringency” refers to the combination of conditions to which nucleic acids are subjected that cause double stranded nucleic acid to dissociate into component single strands such as pH extremes, high temperature, and salt concentration. The phrase “high stringency” refers to hybridization conditions that are sufficiently stringent or restrictive such that only specific base pairings will occur. The specificity should be sufficient to allow for the detection of unique sequences using an oligonucleotide probe or closely related sequence under standard Southern hybridization protocols (as described in J. Mol. Biol. 98:503 (1975)).
As used herein, “endonuclease” refers to enzymes (e.g., restriction endonucleases, etc.) that cut DNA at sites within the DNA molecule.
As used herein, a “restriction endonuclease recognition site” refers to a specific sequence of nucleotides in a double stranded DNA that is recognized and acted upon enzymatically by a DNA restriction endonuclease.
As used herein, the term “nicking” refers to the cutting of a single strand of a double stranded nucleic acid by breaking the bond between two nucleotides such that the 5′ nucleotide has a free 3′ hydroxyl group and the 3′ nucleotide has a 5′ phosphate group. It is preferred that the nicking be accomplished with a restriction endonuclease and that this restriction endonuclease catalyze the nicking of double stranded nucleic acid at the proper location within the restriction endonuclease recognition site.
As used herein, the phrase “modified nucleotide” refers to nucleotides or nucleotide triphosphates that differ in composition and/or structure from natural nucleotide and nucleotide triphosphates. It is preferred that the modified nucleotide or nucleotide triphosphates used herein are modified in such a way that, when the modifications are present on one strand of a double stranded nucleic acid where there is a restriction endonuclease recognition site, the modified nucleotide or nucleotide triphosphates protect the modified strand against cleavage by restriction enzymes. Thus, the presence of the modified nucleotides or nucleotide triphosphates encourages the nicking rather than the cleavage of the double stranded nucleic acid.
As used herein, the phrase “DNA polymerase” refers to enzymes that are capable of incorporating nucleotides onto the 3′ hydroxyl terminus of a nucleic acid in a 5′ to 3′ direction thereby synthesizing a nucleic acid sequence. Examples of DNA polymerases that can be used in accordance with the methods described herein include, E. coli DNA polymerase I, the large proteolytic fragment of E. coli DNA polymerase I, commonly known as “Klenow” polymerase, “Taq” polymerase, T7 polymerase, Bst DNA polymerase, T4 polymerase, T5 polymerase, reverse transcriptase, exo-BCA polymerase, etc.
As used herein, the term “displaced,” refers to the removing of one molecule from close proximity with another molecule. In connection with double stranded oligonucleotides and/or nucleic acids, the term refers to the rendering of the double stranded nucleic acid molecule single stranded, i.e.. one strand is displaced from the other strand. Displacement of one strand of a double-stranded nucleic acid can occur when a restriction endonuclease nicks the double stranded nucleic acid creating a free 3′ hydroxy which is used by DNA polymerase to catalyze the synthesis of a new strand of nucleic acid. Alternatively, one nucleic acid may be displaced from another nucleic acid by the action of electronic biasing of an electrically addressable microchip.
As used herein, “ligase” refers to an enzyme that is capable of covalently linking the 3′ hydroxyl group of a nucleotide to the 5′ phosphate group of a second nucleotide. Examples of ligases include E. Coli DNA ligase, T4 DNA ligase, etc. As used herein, “ligating” refers to covalently attaching two nucleic acid molecules to form a single nucleic acid molecule. This is typically performed by treatment with a ligase, which catalyzes the formation of a phosphodiester bond between the 5′ end of one sequence and the 3′ end of the other. However, in the context of the invention, the term “ligating” is also intended to encompass other methods of connecting such sequences, e.g., by chemical means.
The term “attaching” as used herein generally refers to connecting one entity to another. For example, oligomers and primers may be attached to the surface of a capture site. With respect to attaching mechanisms, methods contemplated include such attachment means as ligating, non-covalent bonding, binding of biotin moieties such as biotinylated primers, amplicons, and probes to strepavidin, etc.
As used herein, the term “adjacent” is used in reference to nucleic acid molecules that are in close proximity to one another. The term also refers to a sufficient proximity between two nucleic acid molecules to allow the 5′ end of one nucleic acid that is brought into juxtaposition with the 3′ end of a second nucleic acid so that they may be ligated by a ligase enzyme.
The term “allele specific” as used herein refers to detection, amplification or oligonucleotide hybridization of one allele of a gene without substantial detection, amplification or oligonucleotide hybridization of other alleles of the same gene.
As used herein, the term “three-prime” or “3′” refers to a specific orientation as related to a nucleic acid. Nucleic acids have a distinct chemical orientation such that their two ends are distinguished as either five-prime (5′) or three-prime (3′). The 3′ end of a nucleic acid contains a free hydroxyl group attached to the 3′ carbon of the terminal pentose sugar. The 5′ end of a nucleic acid contains a free hydroxyl or phosphate group attached to the 5′ carbon of the terminal pentose sugar.
As used herein, the phrase “free three-prime (3′) hydroxyl group,” refers to the presence of a hydroxyl group located at the 3′ terminus of a strand of nucleic acid. The phrase also refers to the fact that the free hydroxyl is functional such that it is able to support near nucleic acid synthesis.
As used herein, the phrase “five-prime overhang” refers to a double-stranded nucleic acid molecule, which does not have blunt ends, such that the ends of the two strands are not coextensive, and such that the 5′ end of one strand extends beyond the 3′ end of the opposing complementary strand. It is possible for a linear nucleic acid molecule to have zero, one, or two, 5′ overhangs. The significance of a 5′ overhang is that it provides a region where a 3′ hydroxyl group is present on one strand and a sequence of single stranded nucleic acid is present on the opposite strand. A DNA polymerase can synthesize a nucleic acid strand complementary to the single stranded portion of the nucleic acid beginning from the free 3′ hydroxyl of the recessed strand.
As used herein, the term “bumper primer” refers to a primer used to displace primer extension products in SDA reaction. The bumper primer anneals to a target sequence upstream of the amplification primer such that extension of the bumper primer displaces the downstream amplification primer and its extension product.
As used herein, the terms “detected” and “detection” are used interchangeably and refer to the discernment of the presence or absence of a target nucleic acid or amplified nucleic acid products thereof.
As used herein, “label” refers to a chemical moiety that provides the ability to detect an amplification product. For example, a label on a nucleic acid may comprise a radioactive isotope such as 32P or non-radioactive molecule such as covalently or noncovalently attached chromophores, fluorescent moieties, enzymes, antigens, groups with specific reactivity, chemiluminescent moieties, and electrochemically detectable moieties.
The above definitions should not be understood to limit the scope of the invention. Rather, they should be used to interpret the language of the description and, where appropriate, the language of the claims. These terms may also be understood more fully in the context of the description of the invention. If a term is included in the description or the claims that is not defined above, or that cannot be interpreted based on its context, then it should be construed to have the same meaning as it is understood by those of skill in the art.
Background Art
Determining the nucleic acid sequence of genes is important in many situations. For example, numerous diseases are caused by or associated with a mutation in a gene sequence relative to the normal gene. Such mutation may involve the substitution of only one base for another, called a “point mutation.” In some instances, point mutations can cause severe clinical manifestations of disease by encoding a change in the amino acid sequence of the protein for which the gene codes. For example, sickle cell anemia results from such a point mutation.
Other diseases are associated with increases or decreases in copy numbers of genes. Thus, determining not only the presence or absence of changes in a sequence is important but also the quantity of such sequences in a sample can be used in the diagnosis of disease or the determination of the risk of developing disease. Moreover, variations in gene sequences of both prokaryotic and eukaryotic organisms has proven invaluable to identifying sources of genetic material (e.g., identifying one human from another or the source of DNA by restriction fragment length polymorphism (RFLP)).
Certain infections caused by microorganisms or viruses may also be diagnosed by the detection of nucleic acid sequences peculiar to the infectious organism. Detection of nucleic acid sequences derived from viruses, parasites, and other microorganisms is also important where the safety of various products is of concern, e.g., in the medical field where donated blood, blood products, and organs, as well as the safety of food and water supplies are important to public health.
Thus, identification of specific nucleic acid sequences by the isolation of nucleic acids from a sample and detection of the sought for sequences, provides a mechanism whereby one can determine the presence of a disease, organism or individual. Generally, such detection is accomplished by using a synthesized nucleic acid “probe” sequence that is complementary in part to the target nucleic acid sequence of interest.
Although it is desirable to detect the presence of nucleic acids as described above, it is often the case that the sought for nucleic acid sequences are present in sample sources in extremely small numbers (e.g., less than 107). The condition of small target molecule numbers causes a requirement that laboratory techniques be performed in order to amplify the numbers of the target sequences in order that they may be detected. There are many well known methods of amplifying targeted sequences, such as the polymerase chain reaction (PCR), the ligase chain reaction (LCR), the strand displacement amplification (SDA), and the nucleic acid sequence-based amplification (NASBA), to name a few. These methods are described generally in the following references: (PCR) U.S. Pat. Nos. 4,683,195, 4,683,202, and 4,800,159; (LCR) EP Application No., 320,308 published Jun. 14, 1989; (SDA) U.S. Pat. Nos. 5,270,184, and 5,455,166 and “Empirical Aspects of Strand Displacement Amplification” by G. T. Walker in PCR Methods and Applications, 3(1):1-6 (1993), Cold Spring Harbor Laboratory Press; and (NASBA) “Nucleic Acid Sequence-Based Amplification (NASBA™)” by L. Malek et al., Ch. 36 in Methods in Molecular Biology, Vol. 28: Protocols for Nucleic Acid Analysis by Nonradioactive Probes, 1994 Ed. P. G. Isaac, Humana Press, Inc. , Totowa, N.J. (Each of the above references are hereby incorporated by reference.)
With respect to analyzing and/or identifying target nucleic acid amplified products, i.e., “amplicons”, other well known techniques have been typically used including comparative size, relative migration analyses (e.g., Southern blot analysis) and hybridization analysis. However, comparative size or relative migration analyses are not optimal because they are undesirably slow and inaccurate. Additionally, while hybridization analysis offers many advantages over these methods, hybridization analysis is neither rapid nor sensitive as compared to the teachings of the present invention.
With respect to PCR technology, since thermal cycling is required, PCR is not optimal for use in a microelectronic environment because the heat fluctuations caused by the thermal cycling are detrimental to the capture sites located on the surface of a microchip. Thermal cycling gives rise to other problems as well including the requirement for complex instrumentation (e.g., to ensure uniform heating, etc.), and, unacceptable time spans for completion of analysis (since each step must occur sequentially).
In contrast to PCR, the SDA technique is useful with microelectronic environments because it overcomes some of the above-described undesirable characteristics of PCR, e.g., it is an isothermal method and the amplification process is asynchronous, and, therefore, more rapid. Although the use of SDA has advantages over PCR, SDA schemes as currently practiced typically include the use of solution-based amplification protocols (e.g., disclosed in the above mentioned U.S. Pat. No. 5,455,166). Recent modifications of the SDA technique have advanced the technique to minimizing the number of individually designed primers for amplification as described in U.S. Pat. No. 5,624,825. However, such advances do not benefit from enhancements realized in the current invention of electronically controlled hybridization.
Other amplification procedures include the use of probes that are bound to a solid support. However, such procedures have not provided a discemable advance in the art compared to the “anchored” SDA presented herein and performed in conjunction with an electronically addressable microchip. For example, U.S. Pat. No. 5,380,489 discloses a method for nucleic acid amplification and detection of target nucleic acids wherein an adhesive element is used to affix capture probes so that target molecules may be more easily captured and detected. This method does not address the issue of simultaneous amplification, capture, and detection as does the current invention. In another example, U.S. Pat. No. 5,474,895 discloses detection of nucleic acids using a polystyrene support-based sandwich assay. Again, such a method merely involves passive hybridization followed by subsequent detection following secondary passive hybridization of a probe.
Microchip arrays have also been used in association with nucleic acid amplification and detection. For example, miniaturized devices have been successfully developed for expression monitoring. See, e.g., M. Schena, et al., 270 Science 467-470 (1995), M. Schena, et al., 93 Proc. Natl Acad. Sci. USA 10614-619 (1996), J. DeRisi, et al., 14 Nat. Genet. 457-60 (1996), R. A. Heller, et al., 94 Proc. Natl. Acad. Sci. USA 2150-55 (1997), and J. DeRisi, et al., 278 Science 680-86 (1997). Miniaturized devices have also been successfully developed for analysis of single nucleotide polymorphisms (SNPs) within an amplicon. See, e.g., Z. Guo, et al., 15 Nat. Biotechnol. 331-35 (1997), and E. Southern, 12 Trends Genet. 110-15 (1996). (Each of the above publications are hereby incorporated by reference). These devices offer the potential for combining the specificity of hybridization with the speed and sensitivity of microchip technology. However, none have successfully provided a suitable miniaturized device for the present purposes.
For example, although micro-devices have been used to analyze multiple amplicons simultaneously (i.e., multiplex analysis), such multiplex analysis has been possible only if hybridization conditions are compatible for each amplicon being tested. This detriment may be partially compensated for by careful capture probe design, by the use of very long captures (e.g. cDNA for expression monitoring) (see, e.g., R. A. Heller, et al., (1997) supra, and M. Schena, et al., (1995) supra), or by extensive redundancy and overlap of shorter capture oligonucleotide sequences. However, taken together, these considerations have imposed limitations on the use of most microchip devices. Moreover, high levels of redundancy such as that used with short oligonucleotide capture sequences results in the requirement for large arrays and complex informatics programs to interpret data obtained, and still certain sequence-specific regions may remain difficult to analyze. Alternatively, the use of long capture oligonucleotides permits use of uniformly elevated hybridization temperatures. However, the use of long capture probes and elevated hybridization temperatures (e.g., in the range of 45 to 75° C.) largely precludes single base pair mismatch analysis of highly related sequences.
Yet another disadvantage has become apparent with conventional microchips (e.g., those disclosed in U.S. Pat. Nos. 5,202,231 and 5,545,531, as well as in E. Southern et al., Genomics 13, 1008-1017 (1992); M. Schena et al., Science 279, 467-470 (1995); M. Chee et al., Science 274, 610-614 (1996); and D. J. Lockhart et al., Nature Biotechnology 14, 1675-1680 (1996) (all of which are herein incorporated by reference)), in that they depend upon passive hybridization and solution based amplification prior to the capture of amplified products on the microchips.
Further, many of these devices are unable to analyze and/or detect the amplification of target molecules from multiple samples simultaneously. In macroscopic devices, this latter problem is conventionally handled by “dot blot” formats in which individual samples occupy unique geometric positions with minimal contamination between samples. In contrast, for most microchips, the problem of detection and analysis usually requires expensive and complex nucleic acid deposition technology similar to dot blot macroscopic deposition but on a microscopic scale.
In another recent disclosure, (PCT WO96/01836), electronic microchips have been used in connection with PCR type amplification of nucleic acids. However, an amplification system requiring the simultaneous use of amplification enzymes and restriction enzymes for increasing the quantity of target amplicons at a specific capture site was not contemplated nor possible in that system. Rather, restriction digestion of captured nucleic acid species was considered in connection with the removal of double stranded nucleic acid species from capture sites following PCR type amplification procedures with detection of target species occurring subsequent to enzymatic cleavage. Moreover, that system provided anchored amplification primers complementary to only one strand of a target nucleic acid that were functional in a PCR reaction.
Like other microchip based amplification and detection platforms, the invention conceptualized in the PCT WO 96/01836 publication is substantially limited as compared to the SDA on electronically addressable microchips disclosed herein because the PCR type amplification of target species as taught in that publication required repeated disruption of double stranded species as well as functionality of solution based reverse primers. Such a situation results in the reduction of efficient amplification due to primer-primer interactions while use of restriction enzymes is inhibited due to fluctuations in reaction buffer conditions.
Finally, other aspects of amplification and detection of nucleic acids have been problematic and/or not optimal. One such problem has been the loss of specificity in the restriction endonuclease cleavage of nucleic acids by restriction enzymes. For example, it is known that some restriction endonucleases lose specificity for their DNA recognition sequence with increased osmotic pressure or reduced water activity. C. R. Robinson et al. J. Mol. Biol. 234: 302-306 (1993). With reduced water activity, the restriction endonucleases will cleave DNA at recognition sites that differ by one base pair from the normal recognition site. The restriction sites that are off by one base pair are called “star” sites and the endonucleases recognition and cleavage of these star sites is called “star activity.”
Robinson et al. found that bound water participates in sequence specificity of EcoRI DNA cleavage (Biochemistry 33(13):3787-3793(1994)), and further found that increasing hydrostatic pressure by conducting the reactions at elevated pressure from 0 to 100 atm. inhibited and ultimately eliminated star activity induced by osmotic pressure for EcoRI, PvuII, and BamHI, but not for EcoRV. (Proc. Natl. Acad. Sci. USA 92:3444-3448 (1995)). One recurrent problem with SDA that relies on restriction endonucleases is the frequency with which non-target sequences are amplified in a primer-independent manner due to star activity. Thus, there is a need to reduce or eliminate star activity in SDA reactions. In one embodiment of the current invention, we provide for the elimination of such star activity in SDA reactions by application of a high pressure SDA method.
In addition to advancing SDA technology by eliminating star activity, we also provide for various other advancements in the detection of nucleic acids using SDA in combination with a bioelectronic microchip. For example, amplification and separation of nucleic acid sequences may be carried out using ligation-dependent SDA. In contrast to ligation-dependent amplification procedures known in the art that require the amplified products to be separated from the starting material by a capture step, or that require that free ligation probe be separated from bound probe prior to ligation, the current invention eliminates the need to make separate isolation steps. Further, the current invention improves upon the SDA amplification process by eliminating the need for bumper primers as they have been used in the art. For example, typical ligation-dependent amplification procedures include capture steps by labeling one of the primers used during amplification. Separation may occur prior to ligation to prevent template independent ligation of the primers or separation may occur following ligation to isolate target DNA amplicons from the non-labeled/amplified DNA. Target DNA amplicons containing this label are separated from the non-labelled/amplified DNA. This separation requires an extra step following amplification. This extra manipulation of the sample increases the complexity of the procedure and thereby renders it less useful as a simple alternative to other current DNA amplification methods such as PCR. This extra manipulation of sample also hinders automation of the amplification procedure. In one embodiment of the current invention a ligation-dependent SDA method is provided that eliminates such extra steps facilitating automation of amplification and detection of target nucleic acids.
In other embodiments, we have provided additional advancements in nucleic acid amplification and detection technology using SDA and electronically addressable microchips which advancements collectively show that a need remains for devices, methods, and compositions of matter for efficiently and optimally amplifying, detecting and analyzing target nucleic acid sequences of interest.